This invention relates to compounds comprising molybdenum, oxygen and certain cations, and the use of these compounds as catalysts in C4 oxidation processes, especially butane oxidation processes.
Oxidative organic processes are widely used in industrial operations. One commercially valuable process involves the oxidation of butane to maleic anhydride. Maleic anhydride is used as a raw material for products ranging from agricultural chemicals, paints, paper sizing and food additives to synthetic resins. To fill the high demand for this valuable chemical, a variety of commercial processes have been developed.
One important route to maleic anhydride involves the vapor phase oxidation of n-butane over a vanadium/phosphorus oxide (VPO) catalyst. The reaction step involves oxidation of n-butane with air (oxygen) to form maleic anhydride, carbon oxides, water and smaller amounts of partially oxidized by-products. Typically, the process is carried out in fixed-bed reactors, fluid-bed reactors, or more recently in recirculating solids reactors having two reaction zones in which two separate reactions take place with a catalyst (the solid) circulating between the two reaction zones and taking part in reactions in both zones.
A number of non-VPO catalysts have been reported in the literature. Zazhigalov, V. A. et al., in an article entitled xe2x80x9cOxidation of n-butane on Vanadium Molybdenum-Oxide Catalystsxe2x80x9d, Inst. Fiz. Khim. im. Pisarzhevskogo, Kiev USSR Neftekhimiya (1977), 17 (2),268-73 describe the activity of V2O5-MoO3 catalysts in butane oxidation as passing through a maximum at 25% MoO3, and that a certain catalytic structure consisting of, V4+, V5+ and Mo6+ ions correspond to their preferred catalyst composition. These results obtained at 500-600xc2x0 C. indicate low catalyst activity at normal operating temperatures.
Mazzochia, C. R. et al., in xe2x80x9cSelective Oxidation of Butane in the Presence of NiO-MoO3 catalystsxe2x80x9d; An. Quim. Ser. A 79, no. 1 108-113(1983) disclose nickel molybdate catalysts prepared by coprecipitation that exhibit low hydrocarbon conversions. At 475xc2x0 C., 19% conversion of n-butane was noted with low selectivities to maleic anhydride.
Umit Ozkan and G. L. Schrader, in xe2x80x9cSynthesis, Characterization and catalytic behaviour of cobalt molybdates for 1-butene oxidation to maleic anhydridexe2x80x9d, Applied Catalysis, 23 (1986) 327-338 disclose the use of cobalt molybdate for the oxidation of 1-butene.
In spite of the progress in catalyst and process development over the years, a need still remains for improved non-VPO catalysts useful in the oxidation of C4 hydrocarbons, particularly n-butane, to maleic anhydride and especially catalysts which are active at lower temperatures and have shorter contact times; and it is to that end that the present invention is directed.
The present invention provides a catalyst, comprising a molybdenum compound of formula I, II, III, IV or V:
I VqMoAyOz 
II NiMoxByOzxe2x80x2
III VNiwMoxCyxe2x80x2Ozxe2x80x3
IV CoNiwMoxDyOzxe2x80x2xe2x80x3
V VNiwCOrMoxEyOzxe2x80x3xe2x80x3
wherein:
q is a number from 0.1 to 10;
r is a number from 0.1 to 10;
w is a number from 0.1 to 10;
x is a number from 0.1 to 10;
y is a number from 0.1 to 10;
y is a number from 0 to 10,
A is at least one cation selected from the group consisting of cations of: Cr, Sb, Co, Ce and Pb;
B is at least one cation selected from the group consisting of cations of: Sb, Al and W;
C is at least one cation selected from the group consisting of cations of: Fe, Zn, Al, Sb, Bi, W, Li, Ba, Nb and Sn;
D is at least one cation selected from the group consisting of cations of: Ba, Mn, Al, Sb, Sn, and W;
E is at least one cation selected from the group consisting of cations of: Fe, Ca, Mn, Sr, Eu, La, Zr, Ga, Sn and Pb; and
z, zxe2x80x2, zxe2x80x3, zxe2x80x2xe2x80x3, and zxe2x80x3xe2x80x3 are determined using the amounts and oxidation states of all cations present in each formula according to the following equations:
z=((q times oxidation state of V)+(1 times oxidation state of Mo)+(y times oxidation state of A)) divided by 2 (oxidation state of oxygen);
zxe2x80x2=((1 times oxidation state of Ni)+(x times oxidation state of Mo)+(y times oxidation state of B)) divided by 2 (oxidation state of oxygen);
zxe2x80x3=((1 times oxidation state of V)+(w times the oxidation state of Ni)+(x times oxidation state of Mo)+(yxe2x80x2 times oxidation state of C)) divided by 2 (oxidation state of oxygen);
zxe2x80x2xe2x80x3=((1 times oxidation state of Co)+(w times the oxidation state of Ni)+(x times oxidation state of Mo)+(yxe2x80x2 times oxidation state of D)) divided by 2 (oxidation state of oxygen); and
zxe2x80x3xe2x80x3=((1 times oxidation state of V)+(w times the oxidation state of Ni)+(r times the oxidation state of Co)+(x times oxidation state of Mo)+(yxe2x80x2 times oxidation state of E)) divided by 2 (oxidation state of oxygen).
The present invention also provides a process for the oxidation of a C4 hydrocarbon to maleic anhydride, comprising: contacting the C4 hydrocarbon with a source of oxygen in the presence of a catalytic amount of a molybdenum catalyst comprising a compound of formula I, II, III or V, as defined above, to yield maleic anhydride.
The present invention further provides a process for the oxidation of n-butane to maleic anhydride, comprising: contacting n-butane with a source of oxygen in the presence of a catalytic amount of a molybdenum catalyst comprising a compound of formula IV, as defined above, wherein the molybdenum catalyst is in a bulk state, to yield maleic anhydride.
The present invention also provides a process for the oxidation of n-butane to maleic anhydride, comprising: contacting n-butane with a source of oxygen in the presence of a catalytic amount of a catalyst comprising a molybdenum compound of formula VI or VII in a crystalline, active phase
VI V9Mo6O40 
VII V2MoO8 
to yield maleic anhydride.
The present invention also provides a process for the preparation of a molybdenum compound comprising a crystalline oxide of formula I, II, III, IV or V, as described above, comprising the steps of: contacting at least one compound having a cation of the molybdenum compound with at least one cation containing compound for each of the other cations of the molybdenum compound in a solution comprising water to form a resultant solution or colloid; freezing the resultant solution or colloid to form a frozen material, freeze drying the frozen material; and heating the dried frozen material to yield the molybdenum compound of formula I, II, III, IV, V, VI or VII.